How are relationships defined and accessed?
Relationships in the DPL are defined using secondary keys. A secondary key provides an alternate way to lookup entities, and also defines a relationship between the entity and the key. For example, a Person entity with a secondary key field consisting of a set of email addresses defines a One-to-Many relationship between the Person entity and the email addresses. The Person entity has multiple email addresses and can be accessed by any of them. Optionally a secondary key can define a relationship with another entity. For example, an Employee entity may have a secondary key called departmentName that is the primary key of a Department entity. This defines a Many-to-One relationship between the Employee and the Department entities. In this case we call the departmentName a foreign key and foreign key constraints are used to ensure that the departmentName key is valid. Both simple key relationships and relationships between entities are defined using the SecondaryKey annotation. This anno
